The present invention relates to a solar battery in which a photoelectric conversion layer for generating a photovoltage is formed by using an amorphous or microcrystalline silicon thin film. Also, the present invention relates to a photoreflective electrode of a solar cell having a texture structure. Further, the present invention relates to a photoreflective electrode of a solar cell using an organic resin film as a substrate.
A solar cell having a photoelectric conversion layer in which at least one PIN junction is formed using a amorphous or microcrystalline silicon film is utilized. In such the solar cell, a light which is incident on the solar cell and transmits without absorbing in a photoelectric conversion layer is reflected by a photoreflective electrode provided in a light incident side position and an opposite side position, and then is incident on the photoelectric conversion layer again, so that a light absorption in the photoelectric conversion layer is promoted by the photoreflective electrode.
The photoreflective electrode is formed by a single layer film (aluminum (Al) or silver (Ag)), a laminate layer of a transparent conductive film (such as indium tin oxide (ITO) alloy, zinc oxide (ZnO), and tin oxide (SnO2)) or a metal film (such as titanium (Ti), chromium (Cr), nickel (Ni), and stainless steel) and aluminum or silver.
A photoreflective electrode has an uneven surface (so called the texture structure) formed by aligning crystals of a metal used as its material at a desired direction. A light which is incident on a photoelectric conversion layer and reaches the photoreflective electrode without absorbing in the photoelectric conversion layer is scattered and reflected at various directions, so that the light is incident on the photoelectric conversion layer. By this, the improvement of light absorbing efficiency is attempted.
When an aluminum film is used as an photoreflective electrode, the aluminum film having the purity of 99.99% is formed on a glass substrate or an organic resin film substrate by sputtering in a conventional case. However, only an aluminum film having almost an even surface can be obtained by this method.
In an uneven aluminum film, since an integral reflection (total amount of total reflection light) includes almost mirror (directional) reflection (a reflection angle of reflection light is the same angle as an incident angle of an incident light), it cannot be expected to utilize a light effectively. Particularly, the long wavelength component of an incident light cannot be utilized sufficiently.
It is attempted to obtain a texture structure by crystal growth of the aluminum film along a desired direction. When aluminum is used as a photoreflective electrode and the aluminum film is formed by sputtering while heating a substrate, crystallization of aluminum is promoted and crystal grains are generated to obtain a surface having an uneven shape. However, since a size and a shape of the generated crystal grains is large or nonuniform for a photoreflective electrode of a solar cell, it is not effective to improve a photoelectric conversion efficiency. Also, a film quality is nonuniform.
In the formed aluminum film, large and abnormal crystal grains having a diameter of about 1 .mu.m may be grown. By generation of the crystal grains, a short circuit may be caused between an upper electrode (light incident side electrode) and a photoreflective electrode. FIG. 5 shows a SEM photograph representing a state that large and abnormal crystal grains are generated in an aluminum film formed by a conventional method.
When an aluminum film is formed on an organic resin film substrate by sputtering, since a film quality is reduced, fine film quality cannot be obtained and a photoreflectivity of a photoreflective electrode is decreased and an electric resistance is increased. As a result, a solar cell having high performance cannot be obtained.